The present invention relates to data storage devices employing magnetic disk data storage media. These media have a layered structure 800 as depicted schematically in FIG. 8, where physical vapor deposition (PVD, or “sputtering”) or another thin-film deposition process may be used to first deposit a magnetic layer 806 with a thickness 816 onto a disk substrate 808, typically made of glass or aluminum. Next, a protective layer 804 having a thickness 814 is deposited on top of the magnetic layer 806—the purpose of protective layer 804 is to prevent oxidation of magnetic layer 806 which could adversely impact the magnetic properties. Finally, a lubricating layer 802 with a thickness 812 is deposited on top of the protective layer 804—lubricating layer 802 protects both the disk and magnetic head (not shown) from damage due to contact with the head slider during disk operation. The spacing between the read/write head contained in an air bearing slider (ABS) and the top surface of the lubricating layer is called the “fly height” (FH) and may be controlled by both aerodynamic factors and thermal fly height control (TFC) as is well known. Note that the actual distance between the read/write heads and the magnetic layer 806 also includes the thicknesses 812 and 814 of lubricating layer 802 and protective layer 804, respectively. Thus variations in the thicknesses of layers 802 and 804 may affect the magnetic writing process which strongly depends on the distance between the read/write head and magnetic layer 806. In addition, magnetic layer 806 may also vary in thickness and magnetic properties as a function of location (radial and azimuthal, roughly equivalent to track and sector) on the disk storage medium due to inevitable process variations during deposition. These variations in the magnetic properties of layer 806 are comprised in the “coercivity”, defined as the reverse field needed to drive the magnetization to zero after the magnetic material has been saturated—the current in the write head required to generate this reverse field is called the “coercive current”. Generally, in order to write a bit of information, a higher, “writing” current will be required to drive the magnetic medium to saturation in the opposite direction—the magnetic flux density needed for writing may be termed the “writing flux density”. The ratio of the writing current to the coercive current is commonly assumed to be constant, often with the notation “k”. Since this ratio is generally assumed to be constant (in the absence of saturation within the write head magnetic circuit), variations in coercivity induce proportional changes in the required writing currents—if the writing current is not changed in response to coercivity variations, the pattern of magnetization may vary, potentially adversely affecting writing widths. The magnetic field at the magnetic layer 806 is controlled by a number of factors including the writing current, Iw, and the “fly height”, FH.
It is typically found that variations in coercivity are relatively “smooth”, i.e., varying over distances of several mm on disks with diameters of a few tens of mm. FIG. 4 schematically illustrates the lowest spatial frequency coercivity variation in a plan view 400 of a disk storage medium—this variation is called “1F”, corresponding to one side 402 of the disk having higher coercivity than the opposite side 406. The origin of the term “1F” is that with this pattern of coercivity variation, the resulting electronic frequency when the disk storage medium is rotating corresponds to the fundamental rotational frequency of the disk, e.g., a 6000 rpm rotation of disk 400 (corresponding to 100 rps) would generate a 100 Hz signal (the frequency “F”). Region 402 with darker shading represents a higher coercivity than region 406 (lighter shading) 180° azimuthally around the disk. Regions 404 and 408 have intermediate coercivities, lower than region 402, but higher than region 406. The coercivity across the disk 400 will vary gradually between the higher and lower values in regions 402 and 406, respectively. Typical magnitudes of the coercivity variation are a few percent (see FIG. 9).
FIG. 5 is a schematic plan view of disk storage medium 500 with a 2F variation in coercivity—in this example, regions 502 and 506 have higher coercivities, while regions 504 and 508 have lower coercivities. Because the coercivity goes through two maxima and two minima azimuthally around the disk, this is termed a “2F” variation, since for a 6000 rpm rotation of disk 500 (100 rps), the resultant electronic frequency would be 200 Hz.
FIG. 6 is a schematic plan view of disk storage medium 600 with a 3F variation in coercivity—in this example, regions 602, 606 and 610 have higher coercivities, while regions 604, 608 and 612 have lower coercivities. Because the coercivity goes through three maxima and three minima azimuthally around the disk, this is termed a “3F” variation, since for a 6000 rpm rotation of disk 600, the resultant electronic frequency would be 300 Hz.
FIG. 7 is a schematic graph 700 of the coercivity 704 as a function of the azimuthal angle (i.e., angle around the circumference) 702 on a disk storage medium for the three examples in FIGS. 4-6. Curves 706, 708, and 710 correspond to FIGS. 4-6, respectively. These are schematic sinusoidal curves which can be compared with actual measured coercivity data in FIG. 9 corresponding to a 2F disk (FIG. 5).
FIG. 1 is a schematic cutaway diagram of a write head 102 (the surrounding air bearing slider structure has been omitted for clarity) and a region 108 of a storage medium where the coercivity is nominal. Flux lines 104 and 106 emerge from head 102 and enter disk 108. The flux density is approximately proportional to the writing current (in the absence of saturation in the magnetic circuit of the write head) and approximately inversely to the fly height. In general, the flux density 106 near the center of head 102 will be higher than the flux density 104 nearer the edges of head 102. Where the flux density is above the writing flux density, medium 108 will be magnetized to produce track 110 with writing width (WW) 112. Outside track 110, flux 104 is below the writing flux density and the magnetic material will not be sufficiently magnetized to record data. FIG. 1 illustrates the nominal writing width 112, resulting from region 108 having the nominal coercivity.
FIG. 2 is a schematic cutaway diagram of the write head 102 as in FIG. 1 and a region 208 of a storage medium where the coercivity is below the nominal value. In this example, where the present invention is not employed, there has been no change to either the writing current or the fly height, thus the distribution of flux lines is essentially unchanged from FIG. 1. However, since the coercivity is lower here, it is “easier” (i.e., the writing flux density, which is proportional to the coercivity, is also lower) to magnetize medium 208 than medium 108 (signified here by wider spacings of the shading lines), thus writing width 212 extends outwards farther on each side into regions where the magnetic flux density from head 102 was too low to magnetize medium 108 in FIG. 1, but is now adequate to magnetize medium 208. As a result, the width 212 of track 210 may be larger than the width 112 of track 110 in FIG. 1.
FIG. 3 is a schematic cutaway diagram of the write head 102 as in FIGS. 1 and 2 and a region 308 of a storage medium where the coercivity is above the nominal value—this represents the opposite situation from FIG. 2, since now medium 308 is more difficult (i.e., the writing flux density, which is proportional to the coercivity, is higher) to magnetize than medium 108 (signified here by narrower spacings of the shading lines). Again, compared with FIG. 1, there has been no change to the flux distribution from head 102, but because it takes a higher magnetic flux density to magnetize medium 308 than medium 108, the width 312 of track 310 may be smaller than the width 112 of track 110 in FIG. 1.
Various methods have been proposed to deal with variations in the coercivities within magnetic storage media. In U.S. Pat. No. 8,009,379 B2, during the writing process the ambient temperature within the disk drive is monitored. Temperature variations can affect the writing process in at least two ways: 1) by affecting the air density directly above the disk surface, the fly height may change due to aerodynamic factors, and 2) the coercivity typically decreases with an increase in temperature. Given the measured temperature, the writing current is varied in a step-wise fashion in an attempt to match the writing current to the writing parameters such as fly height and/or coercivity. No attempt is made to map spatial variations of coercivity across the disk medium to correct for process variations—instead, the average coercivity of the disk at nominal room temperature is determined, and this coercivity is then corrected for temperature fluctuations, either above or below room temperature. Another writing parameter monitored in U.S. Pat. No. 8,009,379 B2 is the degree of “overwrite”, which is a measure of how much residual data remains from any previous writing on an area of the disk—generally it is undesirable to write excessively “hard” (thereby eliminating all residual data), so that a small amount of residual information is preferred. No measurement of the spatial distribution of coercivity on the disk medium is performed prior to writing in the method of this patent, thus undesirable time “overheads” may occur due to the requirement for real-time process parameter monitoring during writing.
In U.S. Pat. No. 6,445,521 B1, a test pattern is written using a plurality of writing currents. The bit error rates (BER) are then measured with the read head intentionally offset from the center of the track where the data is to be stored. The procedure outlined here is laborious, requiring large numbers (e.g., 32) of error rate measurements for each track in order as well as writing operations for noise (with the write head offset) and test patterns (with the head at the nominal track center).
U.S. Pat. No. 7,982,995 B2 discloses an approach for compensating the effects of thermal coercivity variations. The coercivity distribution of a disk storage medium is first mapped by writing data into spots distributed radially and azimuthally across the surface of the disk medium. The writing parameters are then optimized for a nominal room temperature. During writing operations, if the ambient temperature of the storage device is near room temperature, then data may be written at random locations (i.e., anywhere) on the disk medium. Conversely, if the ambient temperature exceeds the nominal room temperature, data may only be written in areas of the disk with higher coercivities when measured at the nominal room temperature. If the ambient temperature is below the nominal room temperature, the opposite procedure is followed and data may only be written in areas having low coercivities when measured at the nominal room temperature. Thus, instead of compensating the writing process for temperature-induced coercivity variations, this method “seeks out” regions whose coercivity will have undergone a thermally-induced change in a direction matching the requirements of a fixed writing process. Obviously, this method may limit writing amounts or speeds, since it imposes undesirable limits to which areas of the disk medium are usable in any particular operational environment.
Given that we have seen that some variation, at least at the few percent level, in the coercivity of the disk storage medium is inevitable, and also given that this variation in coercivity may have deleterious effects on the writing widths of tracks on the disk, it would be advantageous to reduce or eliminate these effects.
It would be advantageous if this control method could be implemented without requiring changes to the physical structure of the read/write head or the arm electronics (AE) which position the read/write head relative to the rotating disk storage medium.